In recent years, there has been a rapid advancement in the development of portable and cordless electronic equipment for consumers. Accordingly, demand is growing for small-size and light-weight secondary batteries with higher energy density to serve as a power source for driving such electric equipment. In particular, non-aqueous electrolyte secondary batteries (e.g., lithium ion secondary batteries), because of their high voltage and high energy density, are expected to grow significantly in the future as a power source for notebook personal computers, cell phones, AV equipment, and the like. Nickel-cadmium storage batteries or nickel-metal hydride storage batteries including an alkaline aqueous solution as an electrolyte, which have been in the mainstream, have been replaced by lithium ion secondary batteries. Most of the non-aqueous electrolyte secondary batteries typically represented by the lithium ion secondary batteries include a porous film made of a polyolefin as a separator interposed between a positive electrode and a negative electrode.
In the non-aqueous electrolyte secondary batteries, there is a need to improve the cycle performance. In a cycle test, in association with repeated charge and discharge of the non-aqueous electrolyte secondary batteries, the battery performance is gradually reduced. One of the causes of the reduction in battery performance is in an expansion phenomenon of the negative electrode. For example, when the negative electrode includes graphite, the graphite absorbs lithium during charge. Accordingly, the negative electrode expands as a result of the charge. In addition, in the interface between the electrode and a non-aqueous electrolyte, a decomposition reaction of the electrolyte occurs as a side reaction during charge and discharge, and gas is generated. Notwithstanding an elevation of battery internal pressure due to the expansion of the negative electrode and the generation of gas, the internal volume of the battery hermetically sealed is not significantly changed. For this reason, a large pressure is also applied to between the positive electrode and the negative electrode.
A separator is interposed between the positive electrode and the negative electrode. The separator is porous and easily compressed or deformed as compared with the positive electrode and the negative electrode. In other words, the separator tends to be easily crushed when the battery internal pressure is elevated. In association with the compression or deformation of the separator, the voids (pore volume) in the separator are decreased. As a result, the amount of electrolyte impregnated into the voids of the separator is decreased, inhibiting the migration of lithium ions. Consequently, the battery resistance is gradually increased in association with the repeated charge and discharge.
Conventionally, the improvement on a separator for better cycle performance has been achieved by optimizing the impedance or air permeability of the separator. However, in reality, the compression or deformation of the separator as described above is considered to have a significant influence on the cycle performance.
In order to improve the cycle performance, one proposal suggests making the compressive modulus of the negative electrode larger than the compressive modulus of the positive electrode or the separator (Patent Document 1).
Another proposal suggests using a composite material including two or more types of polyethylene having different molecular weights by weight for a separator mainly composed of polyethylene (Patent Document 2).    Patent Document 1: Japanese Laid-Open Patent Publication No. 2000-285966    Patent Document 2: Japanese Laid-Open Patent Publication No. Hei 5-25305